This is a renewal proposal for the currently funded one-year R01 grant (EY019466). Visual perceptual learning (VPL) is defined as a long-term enhancement of visual task performance as a result of visual experience, is regarded as a manifestation of visual and brain plasticity and is seen as a promising tool with which to improve vision that has been degraded due to visual disorders. Over the one-year funding period, we have made significant progress in clarifying specificity of VPL. However, we have yet to address an important question: How can the visual system be sufficiently plastic to adapt to environmental changes, but stable enough to protect existing visual information from being replaced with new information? Our long-term goal is to resolve this dilemma in VPL. Developmental studies of vision have resolved the dilemma by setting a window (critical period) during which the visual system is highly plastic in one's early life. This inspires us to investigae the mechanisms of plasticity periods in VPL with adults. We have found two plasticity periods in VPL. The short-term plasticity period may begin during each visual training session and last less than one hour after the end of the session. The long-term plasticity period may last for at least several days, during which a reorganization of visual processing gradually occurs until performance levels for the trained task saturates. Thus, we need to examine the mechanisms of these two periods separately. In Aim 1, we will conduct systematic psychophysical investigations to better understand the two plasticity periods in VPL. We have found that 3-hour monocular deprivation (MD) enhances plasticity of monocular processing possibly by abolishing inter-ocular inhibition, whereas reward priming enhances plasticity of binocular processing in a descending way. We will examine how 3-hour MD and reward priming influence the plasticity and stabilization of these two plasticity periods. In Aim 2, we will examine roles of excitatory and inhibitory processing in the two plasticity periods. The start and end of a visual critical period are associated with changes in the ratio of excitatory to inhibitory signals (E/I ratio). We will address the question of whether and how the two plasticity periods in VPL with adults also relate to E/I ratio changes by means of magnetic resonance spectroscopy. Strong preliminary results have led us to the perspective that the results from Aims 1 and 2 will be consistently and constructively integrated so that functional and neural mechanisms of the two plasticity periods will be better clarified. The results would also be used to develop effective interventions for visual disorders. Although VPL is likely to be associated with changes both within and beyond the visual areas, as our initial approach we will focus on examining visual areas in this proposal.